Résumés

Nannoliths are heterogeneous morphological suites of biogenic carbonate particles with silt-clay size dimensions. They are mainly composed of coccoliths, a suite of calcite complex structures produced by marine coccolithophores belonging to the Haptophyta microalgae division. Coccoliths can be found in marine and coastal facies, and they are used as a palaeoenvironmental proxy and a marine tracer in coastal sediments. Among microscopic biogenic structures commonly observed in tsunami deposits, diatoms are often used to determine the marine provenance and sediment source, but very few investigations about nannoliths were carried out so far. In this paper, we investigate the abundance, distribution and provenance of nannoliths observed in the 2004 tsunami deposits near Lhok Nga (northwest Sumatra, Indonesia) and their implications in terms of sediment sources and flow dynamics. The 2004 tsunami deposits display abundances in nannoliths that are significantly higher (1.0 x 104 to 7.2 x 104 nn/g) than other coastal deposits (4.5 x 103 nn/g), but lower than innershelf deposits (1.3 to 6.9 x 105 nn/g). Such enrichments in nannoliths could represent a tool to recognize palaeo-tsunami deposits. A remarkable characteristic of the Lhok Nga tsunami deposits is their nannolith coastal assemblages despite their relative impoverishment in clay content, which under normal marine hydrodynamic conditions would prevent nannoliths to settle. The abundance of nannoliths in tsunami deposits tends to decrease landward and upward, despite variations due to successive erosion/sedimentation phases by successive waves, and to topographical effects. When coupled with grain-size analyses, the study of vertical trends of nannolith abundances thus represents a complementary data for interpreting tsunami deposits.

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1Estimate the magnitude (or “size”) of past tsunamis after their deposits is one of the major issues to be developed in studies on tsunami hazard assessment. Main limitations are (i) the great variability of tsunami sandy sheets deposited inland (the sediment source and the topography controlling many aspects of the deposition), (ii) the preservation of these soft sediments, and (iii) the distinction between tsunamis and other coastal depositional events (e.g., storms, hurricanes, seiches). A suite of diagnostic characteristics allows palaeo-tsunami deposits to be identified: they are locally extensive and generally finer landward and upwards (from base to top of the sections); distinct upper and lower sub-units can be identified; the lower contact is unconformable or erosional; the clast size varies from boulders to fine silt; there are fossil-rich sub-units (coral and shell fragments); rip-up clasts of reworked material can be found (e.g., soil, roots, wood, debris); clasts fabrics and laminasets may indicate landward, seaward and oscillatory palaeocurrents. These criteria have been confirmed and discussed through recent works on the December 26, 2004 tsunami deposits in Indonesia, Thailand, Sri Lanka and India (e.g., Bahlburg and Weiss, 2006; Goff et al., 2006a; Moore et al., 2006; Hori et al., 2007; Paris et al., 2007; Srinivasalu et al., 2007; Wassmer et al., 2007; Choowong et al., 2008 a and b; Morton et al., 2008).Nevertheless, tsunami sandy deposits analysed for one single event but in distinct areas may display the same grain-size trends and thickness, even if flow depths and velocities were completely different. Indeed, the sediment source and the topography control many aspects of the deposition inland: thickest deposits in the topographic lows, great spatial variations in thickness and upper laminated texture when the sedimentation was limited by steep slopes, very poor sorting and landward coarsening at the wave breaking line, bimodal or plurimodal grain-size distributions reflecting different sources of sediment (e.g., Nanayama and Shigeno, 2006; Paris et al., 2007; Choowong et al., 2008 a and b).Among microscopic biogenic structures commonly observed in tsunami deposits, diatoms are often used to determine the marine provenance and sediment source (e.g., Dawson, 2007), but few investigations about calcareous nannoplankton and other nannoliths were carried out so far (Andrade et al., 2003). Nannoliths are heterogeneous morphological suite of biogenic carbonate particles with silt-clay size dimensions. They are mainly composed of coccoliths, a suite of calcite complex structures produced by marine coccolithophores belonging to the Haptophyta microalgae division (Winter and Siesser, 1994; Billard and Inouye, 2004). In coastal facies, smaller-sized aragonite spicules of particular colonial ascidians can also be present, where most stellar-shape forms belong to the Didemnidae family (Monniot, 1970; Turon, 1986). Coccoliths are one of the main components of deep ocean carbonate oozes (Baumann et al., 2004; Ziveri et al., 2004). However, this group can also be found in coastal facies and thus is used as a palaeoenvironmental proxy and a marine tracer in coastal sediments (Ferreira and Cachão, 2005; Guerreiro et al., 2005; Alday et al., 2006, Drago et al., 2006). In this paper, we investigate the abundance, distribution and provenance of nannoliths observed in the 2004 tsunami deposits near Lhok Nga (northwest Sumatra, Indonesia) and their implications in terms of sediment sources and flow dynamics.

2The Lhok Nga Bay is a 5 km large coastal embayment (25 km²) opened to the west and delimited by calcareous steep slopes (fig. 1). The coast prior to the 2004 tsunami appeared as a continuous beach, breached by small estuaries during the wet season. Main fringing reefs are located between Lampuuk and the Lhok Nga River. The lowest areas correspond to lagoons, swamps and rice crops. Small hills and hummocky terrains refer to dunes, beach ridges and palaeo-dunes reaching 15 m a.s.l. (e.g., Lampuuk, Lam Lho). In the southern part of the bay, the coastal morphology becomes more contrasted, especially south of the harbour, where the coast shows alternating cliffs and flat crescent-shaped bays and creeks.The December 26, 2004 tsunami in Sumatra was one of the largest tsunamis in recorded human history. In Lhok Nga, the tsunami waves were almost 30 m high and runups reached 51 m a.s.l. (Lavigne et al., 2009). Eyewitnesses reported 10 to 12 waves, the second and third ones being the highest. The sea was observed to recede 10 mn after the earthquake and the first wave came from the southwest a few minutes later. The first wave moved rapidly landward as a turbulent flow with depths ranging from 0.5 to 2.5 m from ground. The second and largest wave (i.e., the tsunami bore) came from the west-southwest within 5 mn after the first one and was 15-30 m high at the coast. Few eyewitness’ accounts are available for the outflow (backwash). Both inflow and outflow produced extensive erosion, sediment transport and deposition until 5 km inland (Umitsu et al., 2006; Wassmer et al., 2007; Paris et al., 2009). The erosional imprints of the tsunami extend to 500 m from the shoreline and exceed 2 km along the river beds. The most eroded coasts were tangent to the tsunami wave train, which came from the southwest. The fringing reefs were not efficient in reducing erosion and destruction inland (Baird et al., 2005). R. Paris et al. (2009) estimated that the volume of beach eroded by the tsunami (ca. 1.5 x 105 m³) represents less than 10% of the sediments deposited inland (ca. 1.5 x 106 m³).The sandy deposits laid down by the tsunami inland record the successive inflows and a final outflow (the backwash), as shown by normally graded couplets or triplets of layers (Moore et al., 2006; Paris et al., 2007). The topmost layers, interpreted as the backwash deposition, describe a seaward sequence of increasing mean grain size and decreasing degree of sorting. Composition of the deposits and plurimodal grain size distribution reflect different sources of sediments. The local effects of the topography on depositional conditions could be identified: thickest deposits in the topographic lows (50-80 cm), large-scale spatial variations in thickness and upper laminated texture when the sediment accretion was limited by steep slopes (oscillations), flame structures in the riverbeds, landward coarsening and very poor sorting at the wave breaking point. The tsunami was also able to detach and transport coral boulders in excess of 10 t over 500-700 m and megaclasts from the tidal flat in excess of 85 t over a few metres (Paris et al., 2009). The coincidence of different size modes, from boulders to fine sands suggests that all the material was not transported in suspension, but rather through a combination of rolling, saltation and suspension.The 2004 tsunami deposits in Lhok Nga are particularly rich in bioclasts: coral fragments, coral and sponge triaxones spicules, rhodophyta calcareous algae, nannoliths, diatoms, benthic foraminifers, gastropods, echinoderms and rare bryozoans (Paris et al., 2007). These faunistic and floristic species are typical of a shallow marine environment.

3Samples were selected according to longitudinal transects in the 2004 tsunami deposits (fig. 1; NGA and LHO samples). Additional samples from a pre-tsunami beach (NGA1), and a post-tsunami beach (LPK2), lagoons (LAG) and the inner shelf (m0) were also selected in order to compare compositions and abundances of modern nannolith thanatocoenoses (post-mortem assemblages recently deposited into surface sediments) with the tsunami deposit itself. All samples were prepared following the random settling procedure (Flores and Sierro, 1997) adapted to paralic facies (Ferreira and Cachão, 2003). Due to sample coarseness weights of dried raw sediment varied from 2 to 2.5 g (with 1 x 10-4 g precision) while pipetted volumes varied from 4 to 5 ml. Cover slips were permanently mounted on slides using the synthetic balsam Entellan. For each slide, a 3-cm continuous row was screened through optical cross-polarizing microscope at x 1250 magnification and all specimens tabulated into species whenever possible or into broader taxa. The taxonomic frameworks of K. Perch-Nielsen (1989) and P. Bown (1999) and references therein have been followed. All nannoliths with more than half of the structure preserved were included in the counts.Additionally, the CaCO3 content of the 2004 tsunami deposits was determined using a Bernard calcimeter. This method consists of quantifying the CO2 released when the sample is treated with hydrochloric acid. In a closed system, under a constant pressure and temperature, and without any other gases involved, the quantity of CO32− is directly proportional to the volumetric increase resulting from the release of the CO2. The stratigraphy, internal structure and grain-size analyses of these tsunami deposits are already presented in a previous contribution (Paris et al., 2007).In order to infer the transport capacity of the tsunami offshore, we applied the methodology used for instance by F. Maeno and F. Imamura (2007) and A. Noda et al. (2007). Tsunami waves raised the bed shear velocities to levels above critical values for the entrainment of submarine sediments. We estimated the critical particle size for entrainment based upon the simulated current velocities of the tsunami waves (data provided by A. Loevenbruck et al., 2007). We assumed that the flow type generated by the tsunami is a unidirectional flow, because the wave periods were very long, and that the predominantly sandy sediments behave in an essentially non-cohesive manner (sand ripples).Threshold shear velocity (u*) at the bed can be related to threshold flow velocity U100 (nominally taken as 100 cm above the bed) using Karman-Prandtle equation:

4The 2004 tsunami deposits in Lhok Nga are although almost silty-clay void coarse bioclastic sands. However, they revealed the presence of nannoliths (present day coccoliths and Ascidian spicules), with abundances between 1.0 and 7.2 x 104 per grams(data matrix available at http://mcprojectos.fc.ul.pt/​papers/​Lhok_Nga/​Lhok_Nga_nanno.pdf). These values are one order of magnitude lower than the innershelf present-day sediments off Lhok Nga (from 1.3 to 6.9 x 105 nannoliths per grams). Highest abundance was found in sample LAG6 of the Lampuuk lagoon (8.5 x 105 nn/g). Large amounts of sediment were trapped in this lagoon during the tsunami outflow (or backwash) because the fringing reef acted as a topographical barrier. Sediments collected in LAG6 are medium sand rich in bioclasts, and numerous fragments of centimetric Acropora coral branches. Beach samples have expectable low abundance of nannoliths (ca. 4.5 x 103 nn/g), while bryozoan fragments (fig. 2: images 26-29) and ascidian spicules (fig. 2: images 16-25) are, comparably, more abundant. There is no difference between pre-tsunami (4.4 x 103 nn/g) and post-tsunami beach samples (4.5 x 103 nn/g). Tsunami deposits sampled close to the beach have abundances of nannoliths ten times higher (2.19 x 104 nn/g for NGA4). Simple nannolith diversity (number of distinct non-reworked present day taxa) varies in relation with abundance (r = 0.65). Highest diversities were found for samples NGA-7A and LAG 6, with values similar to diversities obtained for the neritic samples (>10).

5Gephyrocapsids, namely Gephyrocapsa oceanica, a distinct feature of coastal nannolith thanatocoenoses, dominate in all samples (Okada, 1983; Ferreira and Cachão, 2005; Guerreiro et al., 2005). This is corroborated by much less diversity than the oceanic (Indian Ocean) realm (see Andruleit, 2007), lower abundances of small placoliths (Emiliania huxleyi + Gephyrocapsa ericsonii), and the occurrence of ascidian spicules (Alday et al., 2006; Turon, 1986). On the other hand, the common presence of Umbilicosphaera sibogae (fig. 2: image 8) is indicative of warm water assemblages (Winter and Siesser, 1994). Small quantities of reworked forms, mainly Neogene (Dictyococcites, Sphenolithus, Reticulofenestra) and in lesser degree Cretaceous (Watznaueria), were detected which indicatives active processes of erosion from Mesozoic and Cenozoic nearby outcrops (fig. 2: images 30-32). The apparent correlation between extant and reworked forms seems to indicate that the latter were present inside the surface sediments before the 2004 tsnami and not preferentially eroded during the tsunami itself. The coastal and shallow-water signature of the nannoliths observed indicates that there is not an increase of open-ocean over coastal taxa brought by the tsunami.

6Nannoliths, together with other marine biogenic elements (tests of small planktonic foraminifera, sponge spicules, bryozoa colony fragments), could be traced along the 3.5 km long NGA transect in the northern part of the Lhok Nga Bay and compared to samples LHO in the southern part (Paris et al., 2007). LHO3 deposits display abundance (1.4 x 104 nn/g) in the same order of magnitude than NGA samples, but LHO4 has low to very low abundances (from 0 to 4.9 x 103 nn/g). Along the NGA transect (fig. 3), the mean abundances from the shore to 1 km inland are quite high, but remain variable from one site to another (from 1 x 104 nn/g to 5 x 104 nn/g), and then decrease landward (< 2 x 104 nn/g). This longitudinal trend is also observed for the CaCO3 content. Samples NGA 2 to NGA 12 (< 1 km inland) contain more than 25% of carbonates (26-57%), whereas NGA 14 to 18 (from 1.5 km to 3.5 km inland) have less than 25% (3-25%).

7Nevertheless, abundances are highly variable when considered in vertical sections of tsunami deposits. The vertical distribution of nannoliths and the CaCO3 content are not clearly correlated with other sedimentological trends (e.g., thickness, mean size, sorting). Sections NGA2 and NGA 7 (fig. 4) are located respectively 175 m and 330 m from the shore, on the banks of brackish lagoons scoured by the tsunami. These sections thus represent a complete sediment record of the event. R. Paris et al. (2007) used successions of normally graded couplets or triplets of layers to identify the signature of successive waves. Deposits of the first wave were eroded by the subsequent waves, except in some topographic lows (e.g., NGA2-A at 52 cm deep). For both sections, the carbonate content decreases upward, with a slight increase at the arrival of the third wave (NGA2-E and NGA7-D). The second and main wave (i.e., the tsunami bore) is recorded by three layers fining upward (NGA2-B-C-D and NGA7-A-B-C). A first decrease of the number of nannoliths is rapidly followed by a significant increase (NGA2-D and NGA7-C). Same trend occur from NGA7-D and NGA 7-F (third wave?), but with lower abundances, whereas the second decrease upward is continuous along NGA2 section. Finally, the two sections both display a two-phase distribution of nannoliths, but peaks of abundance appear at the beginning of the second phase for NGA 2 (7.2 x 104 nn/g at NGA2-D) and at the beginning of the first phase for NGA 7 (4.5 x 104 nn/g at NGA7-A). A similar vertical trend is observed for section NGA9, located 475 m inland.

8The sediments analysed in this study display nannoliths despite being relatively poor in the silty-clay fraction. Under normal conditions silty-clay poor coastal marine sediments are associated to wave hydrodynamic shore conditions that also prevent nannoliths to settle, what was denominated the High energy gap (Ferreira and Cachão, 2005). Coastal sediments off Lhok Nga, retrieved from 12 m to 18 m water depth, contain significant amounts of nannoliths, which indicates that the High energy gap, if present at Lhok Nga shore, corresponds to water depths less than 10-12 m. Furthermore, unconsolidated silty-clay poor sediments are highly permeable, tending to become rapidly washed out of nannoliths through the percolation of meteoric water. Thus, from a taphonomical and textural point-of-view, a nannolith signature can only be retained by the more clay-rich tsunami deposits. Nannoliths in coarse sediments tend to disappear over time. This reinforces the uniqueness and transitional nature of the newly deposited nannolith assemblages of the Lhok Nga sediments. From a compositional point-of-view, the Lhok Nga sediments are not significantly susceptible of early diagenetic carbonate dissolution by acid waters, due to their biogenic rich carbonate content. However, other tsunami deposits, when mainly constituted of siliclastic sediments (on non-tropical coral reef regions) may experience strong and fast post-depositional carbonate dissolution, particularly for the smaller aragonite spicules. This may also prevent calcareous nannofossils to become registered over time in sediments associated to tsunamis.

9Three other topics need to be discussed: the provenance of the nannoliths observed in the 2004 tsunami deposits and the implicationsin terms of sediment sources; the variations of nannolith abundances related to the morphological setting; the trends of nannolith and carbonate contents along vertical sections.The 2004 tsunami carried a large amount of sediments and debris. Eyewitness accounts recall waves already of black colour before breaking inland (Lavigne et al., 2009). The simulation of threshold shear velocities (fig. 5) confirms that most of the sediments deposited inland came from offshore. Bathymetry off Lhok Nga displays a 20 km large continental shelf that provided great volumes of sediments (fig. 6). In a previous contribution (Paris et al., 2009), we estimated that the volume of beach eroded by the tsunami (ca. 0.15 x 106 m³) represented less than 10% of the sediments deposited inland (ca. 1.5 x 106 m³). Nevertheless, the coastal and shallow-water signature (< 60 m) of the nannolith communities indicates that there is not an increase of open-ocean over coastal taxa brought by the tsunami. Despite high shear velocities of the tsunami over the oceanic basins, the continental talus may have limited sediment transport upward, from the Aceh basin (2700-2300 m deep) to the fore arc shelf (depth < 200 m).Furthermore, the great majority of the nannoliths left inland must have been emplaced and transported by the second and main wave, the impact of the subsequent waves being limited to surface reworking. This is concordant with the upward decreasing abundances observed along the thickest vertical sections. The low abundance of nannoliths in LHO4 samples is probably related to the morphological setting of this area (southern part of the Lhok Nga Bay), where the tsunami runup was limited by steep slopes near the coast. Deposition occurred under oscillatory currents and finest materials were constantly reworked by the successive waves, and then redeposited offshore by the backwash. The morphological setting also explains some vertical trends in abundances of nannoliths, by modifying bottom frictions and roughness, sediment transport modes, and by creating sediment traps.

10We suggest that pulses of nannolith abundance are related with successive waves, rather than with transport modes within each wave. When plotted on a CM diagram (Passega, 1964; Allen, 1971), tsunami samples appear to have been deposited by rolling (bed load) and graded suspension (fig. 6). Rolling is the predominant transport mode. This is also supported by the observed breakages of sedimentation related to gentle slope breaks (e.g., road to Lampuuk at 1 km from the shore), and by the association of sands and boulders deposited simultaneously. Graded suspension occurs during transition phases between successive waves. When coupled with CM patterns (fig. 6), nannolith and carbonate contents are not significantly different from graded suspension to rolling modes. Layers with peaks of nannolith abundances (e.g., NGA2-A, 7-A and 9-A) do not have systematically the same grain-size characteristics.

11The 2004 tsunami deposits in Lhok Nga display abundances in nannoliths that are significantly higher than other coastal deposits, but lower than innershelf deposits. Our study clearly suggests that such enrichments in nannoliths may be used as a tool to recognize palaeo-tsunami deposits. Nannolith preservation in sedimentary deposits is normally a function of their sediment texture and composition. A remarkable characteristic of the Lhok Nga tsunamigenic sediments is their nannolith coastal assemblages despite their relative impoverishment in clay content, which under normal marine hydrodynamic conditions would prevent nannoliths to settle. The abundance of nannoliths in the 2004 tsunami deposits tends to decrease landward and upward, despite variations due to successive erosion/sedimentation phases by successive waves, and to topographical effects. When coupled with grain-size analyses, the study of vertical trends of nannolith abundances thus represents complementary data and evidence for interpreting tsunami deposits. However, more investigations are needed to explain why pulses in sediment mean size, sorting and nannolith contents do not appear simultaneously. After this preliminary study, nannoliths appear as a useful tool in determining the marine provenance and sediment source of tsunamis. Samples analysed should systematically encompass abyssal to coastal zones, especially for large-magnitude tsunamis having high values of threshold shear velocities. The results presented in this paper need to be confirmed by future investigations on other tsunamis, with distinct magnitudes, other high-energy events (e.g., storms, hurricanes), and in different morphological and climatic settings.

AcknowledgementsFunding came from the Délégation Interministérielle pour l'Aide Post-Tsunami (DIPT, project no. 161), the French Embassy in Indonesia and the Centre National de la Recherche Scientifique (CNRS) in France, in the framework of Tsunarisk and ATIP projects coordinated by Franck Lavigne and Raphaël Paris. We are particularly grateful to Pauline Agnew and Johannes Steiger, who corrected the manuscript.

Guerreiro C., Cachão M., Drago T. (2005) – Calcareous nannoplankton as a tracer of the marine influence on the NW coast of Portugal over the last 14000 years. Journal of Nannoplankton Research 27, 2, 159-172.